docs: RTDS is a valid alternative as a scheduler for a cpupool

docs: RTDS is a valid alternative as a scheduler for a cpupool

so update the relevant manpage and the example file.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Juergen Gross <JGross@suse.com>
Cc: Meng Xu <xumengpanda@gmail.com>
Reviewed-by: Meng Xu <mengxu@cis.upenn.edu>
Acked-by: Wei Liu <wei.liu2@citrix.com>